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By the USGS Karoo Basin Province Assessment Team: Christopher J. Schenk, Michael E. Brownfield, Marilyn E. Tennyson, Timothy R. Klett, Tracey J. Mercier, Sarah J. Hawkins, Stephanie B. Gaswirth, Kristen R. Marra, Thomas M. Finn, Phuong A. Le and Heidi M. Leathers-Miller. USGS Fact Sheet 2016–3103, February 2017.
Using a geology-based method, the U.S. Geological Survey estimated a mean of 5.27 trillion cubic feet of undiscovered, technically recoverable coalbed gas in the Karoo Basin Province of South Africa and Lesotho.
The Karoo Basin Province and the coalbed gas assessment unit. Base: National Park Service. Map: USGS.
The basin
The Karoo is a Permian retroarc foreland basin. As the Cape fold belt rose through subduction-related compression, thrust loading bent the crust down into an asymmetric basin, deepest in the south.
- South: the Permian rocks are mostly deep-water shales and sandstones.
- North and northeast: river and delta deposits — conglomerates, sandstones, shales and coals.
The Vryheid Formation
The study focused on the coal-bearing Vryheid Formation of the Ecca Group:
- Structure: several coarsening-upward sequences, each topped by coal, traceable across the study area.
- Thickness: variable; single seams up to 10 meters.
- Rank: high- to low-volatile bituminous, higher in the east.
Lower Permian units of the Karoo Basin. The Vryheid Formation's coals (black bars) are in the northeast. Chart: USGS.
The volcanic catch: Permian sedimentation ended in widespread volcanism as the supercontinent Gondwana broke up. The dolerite intrusions may have damaged the coalbed gas, though interpretations differ. The deeper-water Whitehill and Prince Albert shales hold possible shale gas, which the intrusions may also have cooked off.
The assessment unit
The USGS defined a Vryheid Coal Total Petroleum System and one Karoo Vryheid Coalbed Gas Assessment Unit. Its boundary follows the basin edge in the northeast and the 50-meter thickness contour of the Vryheid Formation in the southwest; mined areas were removed. The model: some gas stayed in the coals after the Mesozoic dolerites intruded. That retention is the main risk.
Thickness of the Vryheid Formation, in meters. Chart: USGS.
Inputs (from U.S. coalbed gas analogs; minimum / mode / maximum, mean):
| Input | Values |
|---|---|
| Potential production area (acres) | 1,000 / 26,449,000 / 41,928,000 (22,792,667) |
| Well drainage area (acres) | 40 / 80 / 120 (80) |
| Success ratio (%) | 5 / 20 / 35 (20) |
| Recovery per well, BCFG (median in place of mode) | 0.04 / 0.08 / 0.3 (0.092) |
| AU probability | 1.0 |
The result
| F95 | F50 | F5 | Mean | |
|---|---|---|---|---|
| Coalbed gas (BCFG) | 1,207 | 4,540 | 11,870 | 5,271 |
Fully risked. F95 means a 95 percent chance of at least that amount. BCFG: billion cubic feet of gas. No natural gas liquids.
The mean, 5,271 billion cubic feet, is about 5.27 trillion. More assessments: the USGS Energy Resources Program.
Sources
Based on Assessment of Permian Coalbed Gas Resources of the Karoo Basin Province, South Africa and Lesotho, 2016, USGS Fact Sheet 2016–3103, U.S. Geological Survey; a work of the United States government in the public domain. The map, the stratigraphic chart, the thickness map and both tables are taken from the fact sheet's PDF; the import had lost them.
Licence : CC0 1.0 (domaine public) · Adapté de pubs.usgs.gov
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